The presence of large deposits of oil shale in the semi-arid, high plateau region of the western United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising a marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products, including hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either mining the kerogen-bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes. According to both of these approaches, oil shale is retorted by heating the oil shale to a sufficient temperature to decompose kerogen and produce shale oil which drains from the rock. The retorted shale, after kerogen decomposition, contains substantial amounts of residual carbonaceous material which can be burned to supply heat for retorting.
One technique for recovering shale oil includes forming an in situ oil shale retort in a subterranean formation containing oil shale. At least a portion of the formation within the boundaries of the in situ oil shale retort is explosively expanded to form a fragmented permeable mass of particles containing oil shale. The fragmented mass is ignited near the top of the retort to establish a combustion zone. An oxygen-supplying gas is introduced into the top of the retort to sustain the combustion zone and cause it to move downwardly through the fragmented permeable mass of particles in the retort. As burning proceeds, the heat of combustion is transferred to the fragmented mass of particles below the combustion zone to release shale oil and gaseous products therefrom in a retorting zone. The retorting zone moves from the top to the bottom of the retort ahead of the combustion zone and the resulting shale oil and gaseous products pass to the bottom of the retort for collection and removal. Recovery of liquid and gaseous products from oil shale deposits is described in greater detail in U.S. Pat. No. 3,661,423 to Donald E. Garrett.
It has been found desirable in some embodiments to have an intact subterranean base of operation above the fragmented permeable mass of formation particles in an in situ oil shale retort. Such a base of operation facilitates the drilling of blastholes into underlying formation for forming a fragmented mass in the retort and facilitates ignition over the entire top portion of the fragmented mass. Additionally, having a base of operation above the fragmented permeable mass of formation particles permits control of introduction of oxygen-supplying gas into the retort, provides a location for testing properties of the fragmented permeable mass, such as distribution of void fraction, and provides a location for evaluation and controlling performance of the retort during operation.
The base of operation is separated from the retort by a layer of unfragmented formation extending between the top boundary of the retort and the floor of such a base of operation. The layer of unfragmented formation is termed a "sill pillar" which acts as a barrier between the in situ oil shale retort and the base of operation during retorting operations. It is, therefore, important that the sill pillar remain structurally sound, both for supporting the base of operation and for preventing entry of heat and gases into the base of operation during the retorting process.
Techniques for forming an in situ oil shale retort containing a fragmented permeable mass of formation particles and having a sill pillar of unfragmented formation between the top of the fragmented mass and an overlying base of operation are described in U.S. Pat. No. 4,118,071 by Ned M. Hutchins and in application Ser. No. 929,250 filed July 31, 1978, by Thomas E. Ricketts, entitled "Method for Explosive Expansion Toward Horizontal Free Faces for Forming an In Situ Oil Shale Retort", now U.S. Pat. No. 4,192,554. U.S. Pat. No. 4,118,071 and application Ser. No. 929,250, now U.S. Pat. No. 4,192,554, are incorporated herein by this reference. The in situ oil shale retort formed by the method disclosed in application Ser. No. 929,250 may not be completely full of oil shale particles, i.e., there can be a void space between the upper surface of the fragmented mass of oil shale particles and the top boundary of the retort.
In other embodiments, the formation overlying the fragmented permeable mass of formation particles extends all the way to the ground surface. In such an embodiment, blastholes are drilled through the overlying formation and ignition of the fragmented mass of particles is accomplished from the ground surface.
Examples of other techniques used for forming in situ oil shale retorts are described in U.S. Pat. No. 4,043,595 by French; U.S. Pat. No. 4,043,596 by Ridley; U.S. Pat. No. 4,043,597 by French; and U.S. Pat. No. 4,043,598 by French et al, each of which is incorporated herein by this reference.
In the past, a variety of techniques have been developed for igniting oil shale particles in an in situ oil shale retort in order to establish a combustion zone. Such techniques are disclosed in U.S. Pat. No. 3,990,835 and U.S. Pat. No. 3,952,801, both by Robert S. Burton, III. According to the techniques disclosed in these patents, a hole is bored to the top of the fragmented permeable mass and a burner is lowered through the borehole to the oil shale to be ignited. A mixture of combustible fuel, such as LPG (liquefied petroleum gas), diesel oil, or shale oil, and oxygen-containing gas, such as air, is burned in the burner and the resultant flame is directed downwardly toward the fragmented permeable mass. The burning is conducted until a substantial portion of the oil shale has been heated above its ignition temperature so that combustion of the oil shale in the fragmented mass is self-sustaining after ignition. Thereafter, oxygen-supplying gas is introduced to the retort to advance the combustion zone through the fragmented mass.
When a retort is formed having a void over the top of the fragmented permeable mass, it is important to ensure that portions of overlying unfragmented formation do not slough into the retort. Sloughing of material from unfragmented formation is increased as the temperature of such unfragmented formation is increased.
When material sloughs into the retort, the time required to ignite such a retort is significantly increased. This results in additional fuel usage, thereby increasing the cost of retorting.
Additionally, when a retort is formed having a sill pillar of unfragmented formation above such a fragmented permeable mass of formation particles, sloughing can cause deterioration of the sill pillar's structural integrity and/or complete structural failure. When a sill pillar fails, gases and heat from the retorting operation can escape into the base of operation, rendering the base of operation uninhabitable, thereby increasing the cost of such retorting operations substantially.
Therefore, during the ignition process, it can be important to minimize heating of the bottom of unfragmented formation overlying the fragmented mass in the retort.
It is also important that an ignition process provide that a combustion zone which is initially formed is not inadvertently extinguished during the process. If, for example, a combustion zone is extinguished, it is estimated that three or more times the amount of fuel can be required to re-ignite the fragmented permeable mass than was originally required. Also, if the combustion zone has advanced more than about ten feet into the fragmented permeable mass and is then extinguished, it may not be possible to re-ignite the retort, regardless of the amount of fuel used since the combusted oil shale above the locus of the combustion zone contains little, if any, combustible material.
Additionally, it can be important that an ignition process provide for forming a combustion zone which has propagated across the entire horizontal cross-section of the fragmented permeable mass of formation particles. Having a combustion zone across the entire horizontal cross-section of the retort enhances uniform retorting and minimizes bypassing of oil shale by the combustion and retorting zones during the retorting process.